Liquid-liquid extraction



United States Patent 2,851,396 LIQUID-LIQUID EXTRACTION Henry S. Myers, Jr., Pasadena, Calif., assignor to C. F.

Braun & Co., Alhambra, Califi, a corporation of California Application September 16, 1954, Serial No. 456,440 6 Claims. (Cl. 19614.52)

This invention relates to improved apparatus and methods for liquid-liquid extraction.

Liquid-liquid extraction is a process in which a solute in a first liquid is extracted from that liquid by a'second liquid intimately mixed with the first. The solute is more soluble in the second liquid than is the first liquid, and preferably the solute is more soluble in the second liquid than in thefirst; The two liquids are of different densities, and ordinarily they are considered mutually insoluble, although actually each one is slightly soluble in the other.

Many conventional liquid-liquid extractors use propeller type mixers in order to obtain the desired mixing of one liquid with the other. Ordinarily the propeller must be mounted in or near the interface'of the two liquids and is powered by a shaft provided with conventional seals and stufiing boxes to ensure a leak-proof mixing zone. 'Power requirements for this type of installation are high and vibration problems are sometimes serious, often to the extent of necessitating excessively heavy and expensive apparatus.

The present invention provides methods and apparatus for liquid-liquid extraction in which propeller-type mixers and the problems accompanying such mixers are eliminated. In addition, the power requirements for conducting the process in accordance with the invention are unusually low. For the purpose of describing this invention the term liquid-liquid extraction is used to mean not only the operation where solute is removed from one liquid by intimate mixing with another liquid but also to include the intimate mixing of two liquids, whether a solute is present or not, to achieve a partial solution or extraction of each by the other. The term solute is used to include materials which may be suspended in the first liquid, as well as materials which are dissolved therein.

In terms of method the invention contemplates liquidliquid extraction in which a first liquid is introduced into a mixing zone with a second liquid. The two liquids are at most only partially soluble in each other, and are of different densities. Gas is bubbled into the mixing zone to produce an intimate mixing of the two liquids. Usually one of the liquids contains a solute, and the mixing is so thorough that a substantial portion of the solute goes into the second liquid. The mixed liquids are then transferred to a settling zone where they separate.

Apparatus for carrying out the invention includes a mixing compartment and means for introducing a first liquid into the mixing zone. Means are provided for introducing a second liquid into the mixing compartment, and means are provided for bubbling gas into the mixing compartment to produce intimate mixing of the two liquids. A settling compartment is connected with the mixing compartment and means are provided for flowing the mixed liquids from the mixing compartment to the settling compartment where the two liquids separate. Means are also provided for separately withdrawing the two liquids from the settling compartment.

In the presently preferred form, the apparatus comprises an upright circular tank having a top and a bottom. The central portion of the tank bottom is perforated and an upright annular partition is disposed around the perforated portion and extends from the tank bottom to Patented Sept. 9, 1958 2 the tank top. The partition thus divides the tank into mixing zone or compartment overlying the perforated portion of the tank bottom and a settling zone or compartment in the annular space between the partition and the tank. A light liquid phase inlet and a heavy liquid phase inlet are provided in the partition near the tank bottom so that light and heavy phases can be introduced into the mixing zone to establish a liquid operating level therein. A mixed-liquid outlet is provided in the partition adjacent the inlets so that the mixed liquids in the mixing zone can be transferred to the settling zone. A first upright batfie is provided in the mixing zone and extends from the partition wall between the inlets and outlet inwardly so that the incoming. liquids must travel around the baffie to reach the outlet. The bafile extends from the tank bottom to a height which is above the liquid operating level in the tank. A second upright bafile dis posed in the settling zone extends from the partition between the outlet and inlets to the tank wall and from the tank bottom to the tank top. The settling zone is provided with a heavy liquid phase outlet and with a light liquid phase outlet. A gas chamber is disposed under the perforated portion of the tank bottom, and means are provided for introducing an inert gas into the chamber under sufficient pressure to force it through the perforations and up into the mixing zone. The top of the tank within the portion is provided with a gas outlet.

These and other aspects of the invention will be more clearly understood from the accompanying drawings in which:

Fig. 1 is a vertical section of a presently preferred form of the apparatus which may be used to conduct liquid-liquid extraction in accordance with the method of the invention; and

Fig. 2 is a view taken on line 2--2 of Fig. 1, partly broken away.

Figs. 1 and 2 illustrate an embodiment of the invention which is provided with three mixing and settling zones, but any desired number may be used.

A liquid-liquid extractor 10 ,comprises an upright cylindrical tank 11 having a side wall 12, a top 13 and a bottom 14. A lower, circular, horizontal tray 15 spaced from the tank bottom, and an upper, circular horizontal tray 16 spaced from the tank top divide the tank into lower, intermediate, and upper chambers 17, 18 and 19, respectively, the chambers being of equal volumes. An annular partition 20 coaxially disposed within the tank and spaced from the tank side wall extends upwardly from the tank bottom through each of the trays to the tank top to form a lower mixing zone 21 and a lower settling zone 22 immediately above the tank bottom, an intermediate mixing zone 23 and an intermediate settling zone 24 immediately above the lower tray. and an upper mixing zone 25 and an upper settling zone 26 immediately above the upper tray. Each mixing zone is within the annular partition and is surrounded by its respective annular settling zone. The portion of the tank bottom and each of the trays within the annular partition are provided with perforations 28 so that, in effect, they are sieve trays. However, other arrangements, such as bubble trays, may also be used.

A light liquid phase supply line 30 extends upwardly through the annular portion of the tank bottom outside the partition and through the partition to deliver fresh light phase to the lower mixing zone just above the perforated tank bottom. A suitable pump 31 is used to supply the light phase to the tank. A heavy phase withdrawal line 32 is provided in the annular portion of the tank bottom between the tank wall and the partition through which extracted heavy phase is withdrawn from the tank.

A gas chamber 34 is attached to the underside of the perforated tank bottom and is supplied gas through a gas inlet 36 by means of a blower 37. A back-pressure valve 38 in a gas outlet 39 in the tank top within the annular partition controls the pressure of the gas within the upper mixing zone. The valve may be omitted if the gas is recycled, as is usually preferred.

The annular portion of the lower tray is provided with a heavy phase downcomer tube 40 which extends through the partition wall directly above the light phase supply line. The intermediate portion of the heavy phase downcomer tube is provided with a gooseneck trap 41 to prevent the light phase from passing up it. The lower tray is provided with a light phase riser tube 42, connecting the upper portion of the lower settling zone to the lower part of the intermediate mixing zone. A heavy phase downcomer 44 is also provided in the upper tray and leading into the intermediate mixing zone directly above the riser tube 42.

A heavy phase supply line 45 extends horizontally through the tank wall into the upper mixing zone just above a riser tube 46 connecting the upper portion of the intermediate settling zoneto the lower portion of the upper mixing zone. A conventional pump 47 connected to line 45 supplies fresh heavy phase to the upper mixing zone under the desired pressure. A light phase withdrawal line 48 is provided in the annular portion of the tank top between the partition and the tank wall. Each mixing zone is provided with a mixed liquid outlet 49 (see Fig. 2) located in the partition adjacent the light and heavy phase inlets and spaced from the inlets in a clockwise direction, as viewed in Fig. 2.

The lower, intermediate, and upper mixing zones are each provided with a separate vertical mixing baille, 50, 51, 52, respectively. The mixing bafiies in the lower and intermediate mixing zones are identically arranged. The lower mixing bafiie extends vertically from the tank bottom to the lower tray and extends inwardly from the partition wall beginning between the light and heavy phase inlets and the mixed liquid outlet along a diameter of the tank to a point near the partition wall; the intermediate mixing battle is similarly disposed within the intermediate mixing zone to extend from the lower tray to the upper tray. The upper mixing baflie is similarly arranged within the upper mixing zone except that it does not extend all the way to the tank top. The baffles in the intermediate and lower zones need not extend from tray to tray but may be stopped at any intermediate height which will be above the operating liquid levels within the respective zones.

The lower, intermediate, and upper settling zones are each provided with a separate settling haille 54. 55. 56, respectively. Each settling bafile begins at the partition wall between the light and heavy phase inlets and the mixed liquid outlet and extends outwardly and curves in a clockwise direction as viewed in Fig. 2 to join the tank wall. The lower settling bafiie extends vertically from the tank bottom to the lower tray, the intermediate settling bathe extends vertically from the lower tray to the upper tray. and the upper settling bafile extends vertically from the upper tray to the tank top. The curvature of the settling zone batlies facilitates the smooth flow of the mixed liquids in a clockwise direction in the settling zone as indicated by the arrows in Fig. 2.

In order to reduce the tendency for liquid to be entrained from one mixing zone to the next, a plurality of horizontal bafiies 58 may be installed in the upper portion of the mixing zones.

In a typical liquid-liquid extraction operation the apparatus is operated as follows: A paratfin oil containing aromatic compounds in solution is introduced into the upper mixing zone as the heavy phase through the heavy phase supply line. Liquid ammonia in which the aromatics are substantially more soluble than is the paralfin oil is introduced into the lower mixing zone through the light phase supply line. A gas, preferably an inert gas such as nitrogen or natural gas, is forced by the blower into the gas chamber and up through the perforations in the tank bottom and each of the trays. The flow of the heavy phase, light phase, and gas are in the directions indicated by the arrows in Figs. 1 and 2. The extractor may be operated with either phase continuous, and for the purpose of this explanation it is assumed that the light phase is continuous.

The gas bubbling up through the perforations in the tank bottom churns the liquid ammonia and paraffin oil supplied by downcomer 40 into a mixture of finely dispersed liquid droplets and bubbles. The intimate mixing of the two phases permits some of the aromatic remaining in the paraffin oil to pass into the liquid ammonia. The mixture passes around the end of the baiiie and out the mixed liquid outlet in the lower mixing zone into the lower settling zone. In the relative quiescence of the lower settling zone the liquid ammonia and parafiin oil again separate into two phases, the heavier paratfin phase being on the bottom. The paraffin is carried from the lower settling zone through the heavy phase withdrawal line for additional extraction or other treatment as desired. The liquid ammonia continues to move upward through the riser 42 into the intermediate mixing zone and is mixed with paratfin oil coming down the downcomer 44 from the upper settling zone. The ammonia and paratfin are mixed and separate as previously described so that the ammonia passes up the riser 46 and the paraliin passes down the downcomer 40. The ammonia then enters the upper mixing zone where it is mixed with the fresh paraffin coming in the supply line. After mixing and settling in the upper zones the ammonia containing the aromatics is taken from the tank through the light phase withdrawal line, and thc paratfin from the upper zone flows down the downcomer 44 into the intermediate mixing zone. The gas which has passed serially through the lower, intermediate, and upper mixing zones is taken from the gas outlet and returned to the blower for re-use.

Under ordinary conditions, for example a temperature of about 30 C., the total pressure carried in the liquid reactor is 175 to 200 pounds per square inch. This pressure is suflicient to keep the ammonia in liquid form at that temperature. The gas pressure drop across the perforation in a particular mixing zone controls the amount of vapor space above the liquid in that mixing zone. This vapor space is necessary to prevent liquid entrainment between trays. Liquid entrainment is further reduced by the horizontal bafiies provided in each mixing zone. For each mixing zone of the extractor the gas pressure drop through the perforation in the tray plus the head of liquid in the mixing zone must equal the hydrostatic head of the liquid in the corresponding settling zone. This is true regardless of the gas rate. At low gas rates the gas pressure drop is small and the depth of the liquid is high. As the gas rate is increased the gas pressure drop also increases and the head of liquid on the tray automatically falls. Thus, the tendency for liquid entrainment to occur as gas rates are increased is offset by the increase in vapor space in each of the mixing zones.

As pointed out above, the power requirements for mixing in the extractor of this invention are low, and only a single blower is required regardless of the number of trays. For a ten-stage extractor with mixing trays three feet in diameter and at eight inch vertical spacing and having inch holes on one inch triangular centers, a 30 horsepower blower is adequate. Under these conditions the inert gas is readily pumped through the extractor at the rate of about 1600 cubic feet per minute. Such an extractor has a liquid throughput of about gallons per minute. At a solvent to oil ratio of two to one, this permits oil to be processed at the rate of about 50 gallons per minute.

Thus, the invention provides both methods and apparatus in which liquid-liquid extraction is accomplished by intimate mixing of two liquid phases with gas agitation which reduces vibration, and construction and operating costs. In the specific arrangement shown, maximum use is made of the space in that the liquids are caused to follow a tortuous path within the apparatus, thus obtaining long effective paths for bothmixing and settling. Although in the apparatus as illustrated the mixing zone is surrounded by an annular settling zone, the arrangement can easily be reversed so that the mixing zone is annular and the settling zone is contained therein, the battles, downcomers, risers, perforations, gas chamber, inlets and outlets being accordingly rearranged.

The method and apparatus of this invention are also useful for any operation which requires intimate contacting of two liquids even when no solute. is involved. For example, it may be used to saturate one liquid with another.

lclaim:

1. In a liquid-liquid. extractor the combination comprising a mixing compartment having a perforated bottom and imperforate side walls, means for introducing a first liquid into the mixing compartment, means for introducing a second liquid into the mixing compartment, the second liquid being substantially insoluble in the first and of a different density, means for forcing gas up through the perforated bottom to produce an intimate mixture of the two liquids, an annular settling compartment disposed around the mixing compartment and having an imperforate inner wall surface forming a barrier preventing gas flow from the mixing compartment to the settling compartment, means for flowing the mixed liquids from the mixing compartment to the settling compartment where the two liquids separate, and means for separately withdrawing the two liquids.

2. In a liquid-liquid extractor, the combination comprising an upright tank having a wall, a bottom and a top, the central portion of the bottom being perforated, an upright imperforate partition disposed around the perforated portion to define within the tank a mixing compartment overlying the perforated portion and an annular settling compartment in the space between the partition and the tank wall, a light phase inlet into the mixing compartment, a heavy phase inlet into the mixing compartment, a mixed liquid outlet opening from the mixing compartment into the settling compartment, bafiie, means for causing liquid to follow a tortuous path in the mixing compartment from the inlets to the outlet,

- a settling compartment heavy phase outlet, a settling compartment light phase outlet, a gas chamber underlying the perforated portion of the tank bottom, a gas outlet in the mixing compartment, and means for introducing an inert gas into the chamber the imperforate partition forming a barrier preventing gas flow from the mixing compartment to the settling compartment.

3. In a liquid-liquid extractor, the combination comprising an upright tank having a wall, a bottom and a top, the central portion of the bottom being perforated, an upright partition disposed around the perforated portion to define within the tank a mixing compartment overlying the perforated portion and an annular settling compartment in the space between the partition and the tank wall, a light phase inlet into the mixing compartment, a heavy phase inlet into the mixing compartment, a mixed liquid outlet in the mixing compartment laterally adjacent the inlets, an upright settling bafiie beginning at the partition between the outlet and the inlets and extending outwardly across the settling compartment to the tank wall, a settling compartment heavy phase outlet, a settling compartment light phase outlet, a gas chamber underlying the perforated portion of the tank bottom, a gas outlet in the mixing compartment, and means for introducing an inert gas into the chamber.

4. In a liquid-liquid extractor, the combination comcompartment adjacent the light phase inlet, a mixed liquid outlet in the mixing compartment near the inlets, an up:

right mixing battle beginning at the partition between the outlet and the inlets and extending upwardly from the tank bottom and inwardly across the mixing compartment stopping short of the partition to leave a passage around the bafiie, a' settling compartment heavy phase outlet, a settling compartment light phase outlet, a gas outlet in the mixing compartment, and means for introducing an inert gas up through the perforations the imperforate partition forming a barrier preventing gas flow from the mixing compartment to the settling compartment.

5. In a liquid-liquid extractor, the combination comprising an upright circular tank having a wall, a bottom and a top, the central portion of the bottom being perforated, an upright annular partition disposed around the perforated portion to define within the tank a mixing compartment overlying the perforated portion and an annular settling compartment in the annular space between the partition and the tank wall, a light phase inlet in the partition near the tank bottom, a heavy phase inlet in the partition vertically above the light phase inlet, a liquid outlet in the partition adjacent the inlets, an upright mixing bafiie beginning at the partition between the outlet and the inlets and extending upwardly from the tank bottom and inwardly across the mixing compart ment stopping short of the partition to leave a passage around the battle, an upright settling baffie beginning at the partition between the outlet and the inlets and extending outwardly across the settling compartment to the tank wall, a settling compartment heavy phase outlet, a settling compartment light phase outlet, a gas chamber underlying the perforated portion of the tank bottom, a gas outlet in the mixing compartment, and means for introducing an inert gas into the chamber.

6. In a liquid-liquid extractor, the combination comprising an upright tank having a wall, a bottom and a top, the central portion of the bottom being perforated, an upright partition disposed around the perforated por tion and extending from the tank bottom to the tank top to define within the tank a mixing compartment overlying the perforated portion and an annular settling compartment in the space between the partition and the tank wall, a light phase inlet into the mixing compartment, a heavy phase inlet into the mixing compartment, a mixed liquid outlet in the mixing compartment laterally spaced from the inlets, an upright settling baffle beginning at the partition between the outlet and the inlets and extending from the tank bottom to the tank top and outwardly across the settling compartment to the tank wall, a settling compartment heavy phase outlet, a settling compartment light phase outlet, a gas chamber underlying the perforated portion of the tank bottom, a gas outlet in the mixing compartment, and means for introducing an inert gas into the chamber.

References Cited in the file of this patent UNITED STATES PATENTS 1,951,787 Child et al. Mar. 30, 1934 2,000,606 Othmer May 7, 1935 2,257,283 Snow Sept. 30, 1941 2,520,391 Findlay Aug. 29, 1950 2,580,010 Fenske et al. Dec. 25, 1951 2,665,975 Ng Jan. 12, 1954 2,729,549 Reman et al Ian, 3, 1956 2,758,057 Luben Aug. 7, 1956 

1. IN A LIQUID-LIQUID EXTRACTOR THE COMBINATION COMPRISING A MIXING COMPARTMENT HAVING A PERFORATED BOTTOM AND IMPERFORATE SIDE WALLS, MEANS FOR INTRODUCING A FIRST LIQUID INTO THE MIXING COMPARTMENT, MEANS FOR INTRO DUCING A SECOND LIQUID INTO THE MIXING COMPARTMENT, THE SECOND LIQUID BEING SUBSTANTIALLY INSOLUBLE IN THE FIRST AND OF A DIFFERENT DENSITY, MEANS FOR FORCING GAS UP THROUGH THE PERFORATED BOTTOM TO PRODUCE AN INTIMATE MIXTURE OF THE TWO LIQUIDS, AN ANNULAR SETTLING COMPARTMENT DISPOSED AROUND THE MIXING COMPARTMENT AND HAVING AN IMPERFORATE INNER WALL SURFACE FORMING A BARRIER 